Java程序辅导

6.092: Intro to Java

3: Loops, Arrays

Assignment 2

Foo Corporation needs a program to calculate 
how much to pay their employees. 
1. Pay = hours worked x base pay 
2. Hours over 40 get paid 1.5 the base pay 
3. The base pay must be no less than $8.00 
4. The number of hours must be no more than 60

Frequent Issues (I)

The signature of the main method cannot be 
modified. 
public static void main(String[] arguments) { 
... 
} 
Frequent Issues (II)

Return values: if you declare that the method is not void, 
then it has to return something! 
public static int pay(double basePay, int hours) { 
if (basePay < 8.0)   return -1; 
else if (hours > 60) return -1; 
else { 
int salary = 0;

...

return salary

} 
} 
Frequent Issues (III)

Don't create duplicate variables with the same 
name 
public static int pay(double basePay, int hours) {

int salary = 0; // OK 
… 
int salary = 0; // salary already defined!! 
… 
double salary = 0; //salary already defined!! 
… 
}

class WeeklyPay { 
public static void pay(double basePay, int hours) { 
if (basePay < 8.0) {
System.out.println("You must be paid at least $8.00/hour");
} else if (hours > 60) {
System.out.println("You can't work more than 60 hours a week");
} else {

int overtimeHours = 0;

if (hours > 40) {

overtimeHours = hours - 40;  
hours = 40; 
}
double pay = basePay * hours;
pay += overtimeHours * basePay * 1.5;
System.out.println("Pay this employee $" + pay); 
}
} 
public static void main(String[] arguments) { 
pay(7.5, 35);
pay(8.2, 47);
pay(10.0, 73); 
}
} 
What we have learned so far

● Variables & types 
● Operators 
● Type conversions & casting

● Methods & parameters 
● If statement 
Today’s Topics 
● Good programming style

● Loops 
● Arrays 
Good Programming Style

Good programming style

The goal of good style is to make your

code more readable.

By you and by others.

Rule #1: use good (meaningful) names

String a1;

int a2;

double b; // BAD!!

String firstName; // GOOD

String lastName; // GOOD

int temperature; // GOOD

Rule #2: Use indentation 
public static void main (String[] arguments) { 
int x = 5;
x = x * x;
if (x > 20) {
System.out.println(x + “ is greater than 20.”);
}
double y = 3.4; 
} 
Have a demo with no indentation 
Ctrl-shift-F to auto-format the file 
Rule #3: Use whitespaces 
Put whitespaces in complex expressions: 
// BAD!!
double cel=fahr*42.0/(13.0-7.0); 
// GOOD
double cel = fahr * 42.0 / (13.0 - 7.0); 

Rule #3: Use whitespaces 
Put blank lines to improve readability: 
public static void main (String[] arguments) {

int x = 5;

x = x * x;

if (x > 20) {
System.out.println(x + “ is > 20.”); 
} 
double y = 3.4; 
} 
Rule #4: Do not duplicate tests 
if (basePay < 8.0) { 
... 
} else if (hours > 60) { 
... 
} else if (basePay >= 8.0 && hours <= 60) { 
... 
} 
Rule #4: Do not duplicate tests 
if (basePay < 8.0) { 
... 
} else if (hours > 60) { 
... 
} else if (basePay >= 8.0 && hours <= 60){ 
... 
} 
BAD 
Rule #4: Do not duplicate tests

if (basePay < 8.0) { 
... 
} else if (hours > 60) {

...

} else { 
... 
} 
Good programming style (summary)

Use good names for variables and methods 
Use indentation 
Add whitespaces 
Don't duplicate tests 
oopsL
Loops 
static void main (String[] arguments) { 
System.out.println(“Rule #1”); 
System.out.println(“Rule #2”); 
System.out.println(“Rule #3”); 
} 
What if you want to do it for 200 Rules? 
Loops 
Loop operators allow to loop through a block 
of code. 
There are several loop operators in Java. 
The while operator 
while (condition) {

statements 
} 
The while operator 
int i = 0;  
while (i < 3) { 
System.out.println(“Rule #“ + i); 
i = i+1; 
} 
Count carefully 
Make sure that your loop has a chance to finish.

The for operator

for (initialization;condition;update){ 
statements 
} 
The for operator 
for (int i = 0; i < 3; i=i+1) { 
System.out.println(“Rule #“ + i); 
} 
Note: i = i+1 may be replaced by i++ 
S
}
Branching Statements 
break terminates a for or while loop 
break; 
ystem.out.println(“Rule #” + i); 
for (int i=0; i<100; i++) { 
if(i == 50) 
r 
Branching Statements

continue skips the current iteration of a loop 
and proceeds directly to the next iteration 
fo (int i=0; i<100; i++) { 
if(i == 50) 
continue; 
System.out.println(“Rule #” + i); 
} 
Embedded loops 
for (int i = 0; i < 3; i++) { 
for (int j = 2; j < 4; j++) { 
System.out.println (i + “ “ + j); 
} 
} 
Scope of the variable defined in the initialization: 
respective for block 
Arrays

Arrays

An array is an indexed list of values. 
You can make an array of any type 
int, double, String, etc.. 
All elements of an array must have the same type. 
Arrays

..

0 1 2 3 .. n-1

Arrays

Example: double [ ]

5.0 2.44 9.01 1.0 -9.9 .. 
0 1 2 3 .. n-1

Arrays 
The index starts at zero and ends at length-1. 
Example: 
int[] values = new int[5]; 
values[0] = 12; // CORRECT 
values[4] = 12; // CORRECT 
values[5] = 12; // WRONG!! compiles but 
// throws an Exception 
// at run-time 
Have a demo with runtime exception 
Arrays 
An array is defined using TYPE[]. 
Arrays are just another type. 
int[] values; // array of int 
int[][] values; // int[] is a type 
Arrays

To create an array of a given size, use the operator new : 
int[] values = new int[5]; 
or you may use a variable to specify the size: 
int size = 12; 
int[] values = new int[size]; 
Array Initialization

Curly braces can be used to initialize an array. 
It can ONLY be used when you declare the 
variable. 
int[] values  = { 12, 24, -23, 47 };

Quiz time!

Is there an error in this code? 
int[] values  = {1, 2.5, 3, 3.5, 4};

Accessing Arrays

To access the elements of an array, use the [] operator: 
values[index] 
Example: 
int[] values  = { 12, 24, -23, 47 }; 
values[3] = 18;     // {12,24,-23,18} 
int x = values[1] + 3; // {12,24,-23,18} 
The length variable

Each array has a length variable built-in that 
contains the length of the array. 
int[] values = new int[12];

int size =      values.length; // 12

int[] values2 = {1,2,3,4,5}

int size2 =     values2.length; // 5

String arrays

A side note

public static void main (String[] arguments){ 
System.out.println(arguments.length); 
System.out.println(arguments[0]); 
System.out.println(arguments[1]); 
} 
Combining Loops and Arrays

Looping through an array 
Example 1: 
int[] values = new int[5]; 
for (int i=0;  i